As general steels for bolts, particularly, JIS standard steels, such as SCM435 or SCM440, are used widely. However, some of these general-purpose steels with a tensile strength of 1,100 MPa or more have the problem that they tend to suddenly cause brittle fracture after fastening for a certain period of time, or so-called delayed fracture. For the purpose of improving the properties for the delayed fracture, that is, the delayed fracture resistance, a steel for high-strength bolts is proposed that has improved the temper-softening resistance.
For example, Patent Document 1 has proposed a technique for improving the resistance to hydrogen embrittlement and the delayed fracture resistance of a steel by dispersing, into the steel, an appropriate amount of a composite compound having a size of 50 nm or less, the composite compound including a Mo-based compound, a Ti-based compound, a V-based compound, or a carbide or a nitride containing two or more kinds of elements selected from the group of Mo, Ti, and V.
Patent Documents 2 and 3 have proposed a technique for improving the delayed fracture resistance by adjusting a chemical composition of a steel and also controlling the conditions for quenching and tempering during manufacturing of bolts as appropriate.